Tagging for object location and command-control-and-communications has potential applications in the areas of arms-treaty verification, battlefield management, precision strike, intelligence gathering, terrain mapping, and emergency response. Synthetic-aperture radar (SAR) has a variety of uses, including surveillance and as the prime sensor for locating and identifying targets. A standard SAR creates a reflectivity map, or image, of the illuminated target area. Image resolution cells one meter on a side and cross-track image swaths of the order of thousands of pixels in extent are readily achievable using current SAR technology. Target features, or patterns, appearing at various places in the image can be located, or positioned, relative to one another, to within a fraction of tile dimension of a resolution cell.
SAR transponders are sometimes placed in the imaged scene to serve as reference targets of known reflectivity, position, and/or polarization. Standard SAR transponders normally retransmit pulses with no modulation; therefore the transponder's echo pattern in the SAR image must compete with all other natural and cultural target clutter, which makes difficult the task of recognizing the transponder and determining its true geographic location within the context of other scene features. The fact that no modulation is used increases the transmit power requirements for the transponder in order to overcome background noise. Most SAR transponders do not send information back to the SAR. Those that do send information back to tile SAR also suffer from the effects of clutter from natural and cultural targets. Finally, standard transponders require two rf antennas, one to receive tile impinging radar pulse and one to retransmit tile echo pulse.
Increased power requirements and the need for two antennas cause standard transponders to be large in size; this limits the number of system applications in which they can be used. A number of overt and covert applications are apparent for light-weight, low-power transponders that can be commanded and precisely located using remote sensors, such as SAR, and that can send status information back to the sensor.
Thus, there is a need for a concept that enhances the transponder's echo relative to the surrounding ground clutter by use of special transponder modulation from a transponder in combination with special SAR signal-processing techniques at the SAR. Such enhancement will allow smaller, lower-power transponders to be built and easily deployed in a variety of application scenarios. Second, there is a need for a method that allows reliable, low-power communication of status information from the transponder to the illuminating SAR. Third, there is a need for simple compact construction of a transponder having only one antenna.
The present invention overcomes the shortcomings not yet addressed by the prior art by providing a novel, active (battery-powered), phase-coded, time-gating transponder and novel SAR signal-processing methods that, in combination, allow recognition and location of the transponder in the SAR image, allow communication of information messages from the transponder to the SAR, and allow the transponder to be constructed with only one antenna. The present invention fulfills the need for a radar transponder concept and SAR signal-processing techniques that enhance the transponder's echo relative to the surrounding clutter or noise. Said concepts and techniques allow the true geographical location of the transponder relative to other features in the scene to be revealed in the SAR image and allow reliable, low-power communications from the transponder to the SAR. The time-gating feature of the present invention, to be described in more detail below, provides a method for making the transponder's transmitting and receiving time intervals mutually exclusive. In this way, the transponder's transmitted echo does not interfere with that same transponder's receiver. This feature allows the transponder to be constructed using only one antenna.